The present invention relates to a touch panel device for detecting the contact of an object such as a finger and a pen with the touch panel device, and more particularly relates to a touch panel device for detecting the contact position of the object by detecting attenuation and cutoff of surface acoustic waves (SAWs) by using excitation elements and receiving elements, each constructed by forming electrodes on a piezoelectric body.
With the spread of computer systems, mainly personal computers, there has been used a device for inputting new information or giving various instructions to a computer system by pointing at a position on a display screen of a display device on which information is displayed by the computer system, with an object such as a finger and a pen. In order to perform an input operation with respect to the information displayed on the display screen of the display device of a personal computer or the like by a touching method, it is necessary to detect the contact position (indicated position) on the display screen with high accuracy.
Well known examples of touch panel device for detecting the contact position of an object such as a finger and a pen are a device using a resistance film, and a device using ultrasonic waves. In the former device using a resistance film, a change in the resistance of the resistance film caused by contact of the object with the resistance film is detected. This device has the advantage of low consumption of power, but has the problems in the aspects of the response time, detection performance and durability.
By contrast, in the device using ultrasonic waves, the contact position of the object is detected by propagating surface acoustic waves on a non-piezoelectric substrate, for example, and detecting attenuation of the surface acoustic waves caused by contact of the object such as a finger and a pen with the non-piezoelectric substrate. There has been developed a touch panel device that uses, as transducers for exciting and receiving surface acoustic waves, comb-like electrodes (IDTs: inter digital transducers) capable of being produced collectively using a photolithography technique. In this touch panel device, as each of the excitation element for exciting surface acoustic waves and the receiving element for receiving propagated surface acoustic waves, an element constructed by forming a comb-like electrode on a piezoelectric body in the form of a thin film is used.
FIG. 1 is an illustration showing the structure of such a conventional touch panel device using comb-like electrodes. In FIG. 1, the numeral 61 represents a rectangular non-piezoelectric substrate, and a plurality of excitation elements 62, each constructed by forming a comb-like electrode on a piezoelectric thin film, for exciting surface acoustic waves are arranged in a line on one end of each of the X-direction and Y-direction of the non-piezoelectric substrate 61. Moreover, a plurality of receiving elements 63, each constructed by forming a comb-like electrode on a piezoelectric thin film, for receiving surface acoustic waves are arranged in a line on the other end of each of the X-direction and Y-direction of the non-piezoelectric substrate 61 so that the receiving elements 63 face the excitation elements 62.
In this touch panel device, periodic signals are inputted to the excitation elements 62 to excite surface acoustic waves and propagate them on the non-piezoelectric substrate 61, and then the propagated surface acoustic waves are received by the receiving elements 63. Moreover, when an object such as a finger and a pen comes into contact with the propagation path of the surface acoustic waves on the non-piezoelectric substrate 61, the surface acoustic waves attenuate. Accordingly, it is possible to detect the presence or absence of contact of the object and the contact position by detecting whether or not the level of the received signals at the receiving elements 63 is attenuated.
Furthermore, in order to achieve continuous detection of the presence or absence of contact of an object and the contact position and increase the time difference to improve the resolution of the detected position, the present inventor et al. have proposed a touch panel device in which the excitation elements and receiving elements are disposed so as to propagate surface acoustic waves in oblique directions (diagonal directions) of the non-piezoelectric substrate.
FIG. 2 is an illustration showing the structure of an example of such a touch panel device (hereinafter referred to as the “first prior art example”). FIG. 3 is an enlarged sectional view of FIG. 2. In FIG. 2, the numeral 71 represents a rectangular non-piezoelectric substrate, and a center portion enclosed by an alternate long and short dashed line is a detection region 71a capable of detecting the contact position.
In a frame region 71b outside the detection region 71a, which is a peripheral section of the non-piezoelectric substrate 71, excitation elements 72 are disposed on the upper side and lower side of the substrate 71, while receiving elements 73 are positioned on the left side and right side thereof. FIG. 4 is a partial cross sectional view of the excitation element 72 or the receiving element 73, and the excitation element 72 or the receiving element 73 is constructed by forming a comb-like electrode 75 on a piezoelectric body 74 in the form of a thin film. This comb-like electrode 75 comprises facing bus electrodes 77 and a plurality of electrode fingers 78 which are extended from the bus electrodes 77 in turns and bent in the middle. The structure having such electrode fingers 78 bent in the middle is also called a “chevron structure”. According to this structure, lines of a plurality of electrode fingers 78 tilted in two directions from the facing direction of the bus electrodes 77 are formed, and thereby realizing excitation of surface acoustic waves in two directions and reception of surface acoustic waves from two directions.
Terminals 79 for input and ground are provided so that they are connected to the bus electrodes 77 of the upper-side and lower-side excitation elements 72. Moreover, terminals 79 for output and ground are provided so that they are connected to the bus electrodes 77 of the left-side and right-side receiving elements 73. Further, drawn-round wires 80 are connected to the terminals 79, respectively.
In such a structure, surface acoustic waves are excited in two directions by the excitation elements 72, and the excited surface acoustic waves are propagated in two diagonal directions of the non-piezoelectric substrate 71 and then received by the receiving elements 73. Based on the received results, the presence or absence of contact of an object and the contact position are detected in the same manner as in the conventional example shown in FIG. 1.
Furthermore, the present inventor et al. have proposed a touch panel device comprising a film-like piezoelectric body, a comb-like electrode formed on one surface of the piezoelectric body, and a plate electrode formed on the other surface of the piezoelectric body (hereinafter referred to as the “second prior art example”). In this second prior art example, each of the excitation element and receiving element is constructed by forming a comb-like electrode produced by extending a plurality of electrode fingers which are bent in the middle and have the same polarity on one surface of the piezoelectric body, instead of a comb-like electrode in which electrode fingers of different polarities are placed in turns, and by forming a plate electrode with a polarity different from the comb-like electrode on the other surface of the piezoelectric body. Compared to the first prior art example which requires a width of two lines of bus electrodes on each side due to the presence of the electrodes of different polarities on the same plane, the second prior art example requires only a width of one line of bus electrode, thereby making it possible to narrow the frame region. In addition, since there is no need to provide electrodes of different polarities on the same plane, it is possible to improve the degree of freedom in design of the drawn-round wires, etc. This electrode structure of the second prior art example is also called an “SPT (Single Phased Transducer) electrode structure”.
In the first and second prior art examples of propagating surface acoustic waves in diagonal directions, the propagation distance of the surface acoustic waves is not uniform. Since the surface acoustic waves are attenuated by the propagation, a high-level received signal is obtained by the receiving element in a region with a short propagation distance between the excitation element and the receiving element. However, in a region with long propagation distance between the excitation element and the receiving element (region near the diagonal of the substrate), the surface acoustic wave is attenuated considerably and there is a problem that a high-level received signal is not obtained by the receiving element. As a result, the S/N ratio in the region near the diagonal decreases and high detection accuracy is not obtained for the detections of presence or absence of contact of an object and the contact position, and thus there is room for improvement.